Centered strap spacer for electrodialysis unit



June 23, 1959 E. AQMASON ET AL 2,891,899

CENTERED STRAP SPACER FOR ELECTRODIALYSIS UNIT Filed June 13, 1957 xx? mfgg q/ i I II il f x 'J'L-- :15 a

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United States Patent CENTERED STRAP SPACER FOR ELECTRO- DIALYSIS UNIT Edward A. Mason, Lexington, Charles E. Tirrell, Nahant, and Thomas A. Kirkharn, Waltham, Mass, assignors to Ionics, Incorporated, Cambridge, Mass, a corporation of Massachusetts Appiication June 13, 1957, Serial No. 665,489

12 Claims. (Cl. 204301) The present invention relates to electrodialysis apparatus whereby improved turbulence and mixing of the liquids flowing through the chambers of the electrodialysis chambers are obtained. More particularly, it deals with the shape and structure of straps located in the hydraulic flow path in the chambers of an electrodialysis unit wherein the straps do not contact the adjacent membranes defining said chambers to the effect that an increased mixing or turbulence takes place in saidliquid flow path in said chambers. One aspect of the invention comprises an improvement upon the apparatus disclosed in U.S. Patent No. 2,708,658, issued May 17, 1955, to N. W. Rosenberg.

In the patent referred to above there is disclosed apparatus comprising essentially a central chamber and two end electrode chambers, or two end chambers defined between a plurality of alternating selectively permeable anion and cation exchange membranes. When a D. C. voltage is impressed upon the electrodes one of the end electrodes becomes an anode, and the other a cathode. All the chambers contain flowing electrolyte solutions therein. Thus on passage of D. C. current through this assembly, cations and anions are respectively carried by the current through the selective ion permeable membranes whereby the electrolyte solutions in the diluting chambers are demineralized, at least in part, and the electrolyte solutions in the alternating concentrating chambers become more concentrated. Electroneutrality is preserved in the end chambers by electrode reactions.

In operating apparatusof the type described it was found that, because of the change in transport number occurring at the interface of the solutions and ion exchange membranes, so-called polarized films formed adjacent to the inner surface of both type membranes. To overcome the deleterious effects of polarization it was known to separate the ion exchange membranes by electrically and chemically inert spacers with cut out portions therein to form various shaped tortuous channels which comprise the chambers of the electrodialysis unit containing the flowing electrolyte solutions. While the tortuous path spacer showed an improvement in the operation and efficiency of electrodialysis systems the present invention of employing centered straps, namely, straps in the spacers or dividers of the chambers centrally located which straps provided a much greater improvement for the reasons best understood by the following.

In the operation of ion transfer membranes the capacity for salt removal (or conversion in the case of reactors) is directly related to the current density which is applied to the equipment. The higher the current density the greater the rate of passage of electricity passed through the equipment; therefore the greater is the rate of chemical conversion. It has been found in electrodialysis equipment, however, that there is usually a limiting current density beyond which the increases in current produce side reactions, many of which are undesirable.

For example, in the removal of salts, such as sodium chloride, from water, the transfer of salt, NaCl, from one water stream to another with multiple membrane electrodialysis equipment, use of high current densities is desirable to increase equipment capacities. Beyond a certain current density the increases in current density result in minor increases in the rate of sodium and chloride transfer and increasing rates of transfer of hydrogen and hydroxyl ions from the diluting compartments. This transfer of hydrogen and hydroxyl ions is made evident by pH changes in both streams. The reason for these pH changes is that at high current densities the rate of mass transfer of salt (e.g. sodium and chloride) ions up to the membrane interface by both electric and mass transport is insufficient to carry all of the electric current. The transfer films adjacent to the membranes become depleted in sodium and chloride and hydrogen and hydroxyl ions then move through the membrane carrying the current. These hydrogen and hydroxyl ions are supplied by the disassociation or splitting of water.

This phenomenon is referred to as polarization. Polarization if a cation transfer membrane results in the transfer of hydrogen ion through the membrane into the concentrating stream such that the concentrating stream becomes more acidic and the diluting stream turns basic, due to the hydroxyl which remains in the diluting compartment equivalent to the hydrogen which is transferred through the membrane. On the other hand polarization of an anion membrane results in transfer of hydroxyl ion through the membranes leaving hydrogen ion behind in the diluting compartment. Thus the diluting compartment becomes acidic and the concentrating compartment becomes basic. In natural waters containing calcium and bicarbonate ions, the polarization of anion transfer membranes can result in serious difficulties due to the fact that the bicarbonate ion in the concentrating stream and that being transferred through the membrane under polarizing conditions is converted to carbonate ion by the reaction of bicarbonate and hydroxyl. This, in the presence of calcium, results in the formation of insoluble calcium carbonate precipitate which plugs the membrane and adjacent concentrate passages and causes a rapid increase in the electrical resistance of equipment, and, eventually, physical deterioration of the anion transfer membrane itself. In the operation of water demineralization equipment on such natural waters it is therefore important that the current density applied to the unit be such that polarization does not occur to any significant extent or this deleterious precipitation will occur.

It has been observed that there is arelationship between the permissible current density, up to which polarization is not significant, and the concentration of the salt in the diluting compartment. That is to say, at high salt concentrations, there is more salt available for carrying the current and consequently the permissible current density is higher. This relationship between current density and normality can be expressed in terms of the ratio of CD/N, where CD is the applied current density and N is the normality of the salt at any point in the dilute stream passage.

The limiting polarization depends not only upon the salt concentration but also upon the turbulence and mixing which is present in the flow channel. High mixing or turbulence supplies more salt to the membrane interface so that the permissible current density under conditions of high turbulence is higher than under conditions of low turbulence. Turbulence is affected by the design and construction of the flow path as well as by the velocity of the solution flowing through the diluting compartment and the physical properties of the diluting stream itself, such as viscosity and density.

The present invention consists of the use of shaped members, such as straps, located in the hydraulic flow path of electrodialysis units, wherein these straps do not touch the permselective membranes but are separated from the membranes by a definite distance or clearance to thus increase the mixing or turbulence which takes place in the fiow path. These straps which function as turbulence promoters can be of various sizes and shapes and the spacing between the membrane and the near edge of the strap can be varied, but the essential principle of the present invention is that there is always a definite clearance between the straps themselves, and a definite clearance between the straps and the membranes.

The shape and size of the straps may be varied and may take the cross-sectional form, for example of rectangular, round, streamline, etc., they may vary in thickness, width, distance apart, and clearance from the membranes depending upon the various conditions of flow. The straps are joined to or an integral part of material (spacer) Which support the membranes and which form the channel dividers. Although the straps of the present invention are usually substantially centered in the flow channel of the spacer, it is also contemplated that the spacer may be located nearer to the polarizing membrane when such a situation is present. It will be apparent that spacers having the straps of the present invention may be made by molding, laminating three or more layers, the inside layer or layers having the straps, and other ways well known in the art.

The novel concept in the present invention is based upon the fact that no part of the membranes is subjected to dead spots caused by contact between strap material and membrance. It has been found in the past that the effects of polarization, such as precipitation, have been located particularly in the downstream region immediately behind straps which touch the membranes. In this region apparently the mixing and scouring action of the solution across the membran surface is inferior to that in the area in which there was no strap touching the membrane. Furthermore, experimentation has shown that in the diluting compartment of a demineralizer the polarization limits, namely, the value of permissible ratio of current density to normality (which is used as a measure of polarizability), increased from the cases where the straps touch the polarizing membrane in each cell pair to the case where the strap touched the nonpolarizing membrane; and finally the higher values are reached when the strap is located in the channel away from both membranes. Furthermore, in the present cen tered strap-spacer the membrane surface at no point underneath the strap is blocked from the passage of current. In the use of standard prior spacers where the straps touch the membrane, essentially no current can flow, whereas in the present centered strap-spacer some of the current can flow in the sight shadow cast by the strap itself. This, in effect, gives a higher efi'ective for current flow which is advantageous in incr ming capacity of equipment. Appreciably higher .es of CD/N have been achieved in using the present centered strap spacer over that of the conventional spacer where the straps or supports alternately touch the anion and cation membranes defining the flow chambers.

This invention could also have commercial application in the field of heat transfer where the bounding membranes are replaced with metal heat conductive sheets and the solutions passing through the flow chambers of the spacers are replaced with hot fluids such as steam, hot water, etc. The principle of promoting turbulence in the flow of the hot fluid to enhance the efficiency of heat transfer therefrom to outer areas is contemplated as another commercial and useful application of the present invention.

The primary object of the invention is to increase the range of permissible current densities for a given concentration in which ions may be removed from solutions under optimum current and voltage efiiciency.

Another object of the invention is to separate the function of supports for spacers and turbulence promoters in the hydraulic flow paths of an electro-dialysis unit.

Another object of this invention is to increase the capacity of an electrodialysis unit by allowing for higher permissible values for current densities.

Still another object of this invention is to minimize the thickness of stagnant films of solution at the surfaces of the membranes.

These and other objects and features of the invention will be more readily understood and appreciated from the following detailed description of preferred embodiments thereof selected for purposes of illustration and shown in the accompanying drawings in which:

Figure l is a plan view of a single chamber showing a portion broken away to reveal structural details,

Figure 2 is a cross sectional view along line 22 of Figure 1,

Figure 3 is a cross sectional view along line 3-3 of Figure 1,

Figure 4 is an enlarged view of a strap in the flow channel of Figure 2 to illustrate dimensional relationships thereof.

Figures l, 2 and 3 are different views showing the combination of a single spacer 1 of the present invention in face-to-face contact with two membranes, 2 and 3, Figure 1 being broken away to illustrate the straps 4 in the flow channel 5 of the spacer member lt. Channel S is provided with inlet 6 and outlet 7 for flow of liquid therethrough. In practice a plurality of such spacer-membrane combinations are employed in stacked relationship with end electrode chambers for passing a direct electric current therethrough, the latter being common practice and not shown herein. The center-t0- center spacing of centered straps 4 is shown in Figure 1 as D4, and the width of the channel 5 as D5.

Figure 2 clearly shows the principal feature of the invention wherein the centered straps 4 are shown to be situated away from membranes 2 and 3 with restrictive passageways 8 and 9 to alow flow of solutions to sweep around the straps 4 to effectively promote turbulence and mixing of said solution in contact with said membranes.

Figure 3, a cross sectional View of one of the straps, shows the restrictive passageways or clearances 8 and 9 in relation to the restrictive strap 4 of spacer ll.

Figure 4 is an enlarged view of one of the centered straps 4 in channel 5 shown in Figure 2 wherein D1 represents the thickness of the strap, D2 and D3 the clearances between the strap and the membranes adjacent thereto, and D6 the thickness of the flow channel. D7 represents the width of the strap. While D2 and D3 are usually about the same value, this may not always be true as for example when a polarizing membrane is present, in which case the strap is situated off center nearer to said polariing membrane than the nonpolarizing membrane.

It will be apparent from the drawings that the variables involved in the elements of the present structure are strap spacing D4, strap Width D7, strap-to-membrane clearances D2 and D3, strap thickness D1, in combination with channel thickness D6 and channel width D5.

Whereas the broad principle of the present invention has been set forth to provide a definite clearance of the centered strap between said strap and the adjacent membranes, it has been found that generally best results are obtained when the distance, center-to-center, between adjacent spaced straps is substantially greater than the width of said straps, and more specifically, from two to twenty times the strap width with optimum spacing at six times the strap width. Also the clearance of the strapto-membrane is less than the width of the straps, as for example, about the strap width.

More specifically the following ranges of values of the essential variables have been found for best results to be as follows:

(a) Clearances between the edges of the straps and the adjacent membranes in the range of about 0.010 to about 0.050 centimeter, with an optimum about 0.030 centimeter.

(b) Straps spaced apart, center-to-center, in the range of about 0.5 to about 3.0 centimeters, with an optimum about 1 centimeter.

(c) Width of the straps in the range of about 0.05 to 0.30 centimeter, with an optimum about 0.10 centimeter.

(d) Strap thickness is of no real importance providing the clearance noted above are maintained.

One illustrative embodiment of this invention is a standard 18 x 20 inch membrane stack wherein the straps of the spacers between said membranes are approximately 0.2 centimeter wide by approximtely 0.070 centimeter thick, attached centrally to the spacer of about 0.130 centimeter in thickness in the flow channel between anion transfer and cation transfer membranes in a multiple chamber electrodialysis unit. The center of the strap is located in the center of the passage or flow path formed by said cation and anion transfer membranes. The edges of the strap are about 0.030 centimeter from each membrane forming the clearances. While rather narrow clearances between the edges of the strap and the membranes are necessary for promoting turbulence and mixing of the flow solution, too low a clearance is also undesirable since plugging may result from a filtering action on suspended solids in the solution. Generally, strap to membrance clearance of about 0.015 to 0.080 centimeter for channel thickness of about 0.10 to 0.24 are effective. The straps are spaced, center-to-center, approximately 1.2 centimeters apart. The flow channel width is about 1. centimeter, but this latter element is of no critical consideration here except that it is desirable to have as wide channels as possible so as to minimize the area blocked off by the channel dividers which offer support for the membranes. However, too wide a channel would permit excessive bowing and distortion of the membranes which may cause structural failure of the membranes.

This invention has been described in detail with reference to specific preferred embodiments thereof, but it is contemplated that modification thereto will occur to those skilled in the art, and that such modifications may be made without departing from the scope of the invent1on.

Having thus disclosed our invention and described in detail preferred embodiments thereof, we claim and desire to secure by Letters Patent:

1. A spacer member comprising a sheet of dielectric material having a channel portion therethrough and extending longitudinally of the faces of the sheet with inlet and outlet means at the channel ends thereby providing a flow path therein, said spacer member having a plurality of spaced flow-restricting straps substantially centered in said spacer member and extending across said flow path parallel to the faces of said spacer member, said straps having a thickness less than the thickness of the spacer member thereby providing restrictions in the flow path and which do not extend to the parallel outer faces of said spacer member.

2. A spacer member having at least one continuous channel therethrough forming a chamber for liquid flow therethrough and extending longitudinally of the faces of the sheet, spaced parallel straps running across said chamber in a direction to oppose flow through said chamher, said straps lying substantially in a plane parallel to the faces of said spacer member, said straps having a thickness less than the thickness of the spacer member and which do not extend to the parallel outer faces of said spacer member, and an inlet at one end and an outlet at the other end of said channel therefor.

3. The spacer member of claim 2 in which the distance between adjacent spaced straps is substantially greater than the width of said straps.

4. The spacer member of claim 3 wherein the straps are spaced apart in the range of two to twenty times the strap width.

5. The spacer member of claim 2 in which the continuous perforation presents a tortuous path through said spacer member.

6. The spacer member of claim 3 wherein the straps are spaced apart, center-to-center, in the range of about 0.5 to about 3.0 centimeters.

7. The spacer member of claim 6 wherein the straps are spaced apart, center-to-center, about 1 centimeter.

8. An electrodialysis apparatus comprising end electrodes and a plurality of ion selective membranes therebetween the combination therewith of spacer members interposed between and engaging said membranes in faceto-face contact therewith, said spacer members comprising sheets of dielectric material each having at least one continuous channel therethrough and extending longitudinally of the faces of the sheet to form flow chambers, means for passing liquids through said chambers including an inlet at one end and an outlet at the other end thereof, said spacer members having a plurality of spaced centered straps across said chambers whereby clearances are provided between said straps and each of the adjacent membranes for promoting turbulence and mixing of liquids passing therethrough 9. Apparatus for transferring ions of one solution to another comprising a plurality of perforated spacer members each disposed in face-to-face contact on each side with selectively ion permeable membranes, said spacer members having a continuous channel therethrough and extending longitudinally of the faces of the sheet to form flow path chambers in contact with said membranes, means for introducing a solution into one end of each said chamber, means for removing solution from the other end of each said chamber, and means for flowing a direct electric current transversely through the mem branes and chambers; the improvement comprising a plurality of spaced centered straps in said spacer members running across said chambers in parallel to the faces of the spacer members and in a direction to oppose flow through said chamber, said straps having a thickness less than the thickness of said spacer members whereby clearances are provided between said straps and each adjacent membrane for promoting turbulence and mixing of said solutions in the flow chambers.

10. The apparatus of claim 9 wherein the clearances between the straps and the adjacent membranes are in the range of about 0.10 to about 0.050 centimeters.

11. The apparatus of claim 10 wherein the clearances between the straps and the adjacent membranes is about .030 centimeter.

12. The apparatus of claim 9 wherein the width of the straps is in the range of about .02 to 0.20 centimter.

References Cited in the file of this patent UNITED STATES PATENTS 2,784,158 Bodamer et a1. Mar. 5, 1957 

9. APPARATUS FOR TRANSFERRING IONS OF ONE SOLUTION TO ANOTHER COMPRISING A PLURALITY OF PERFORATED SPACER MEMBERS EACH DISPOSED IN FACE-TO-FACE CONTACT ON EACH SIDE WITH SELECTIVELY ION PERMEABLE MEMBRANES, SAID SPACER MEMBERS HAVING A CONTINUOUS CHANNEL THERETHOUGH AND EXTENDING LONGITUDINALLY OF THE FACES OF THE SHEET TO FORM FLOW PATH CHAMBERS IN CONTACT WITH SAID MEMBRANES, MEANS FOR INTRODUCING A SOLUTION INTO ONE END OF EACH SAID CHAMBER, MEANS FOR REMOVING SOLUTION FROM THE OTHER END OF EACH SAID CHAMBER, AND MEANS FOR FLOWING A DIRECT ELECTRIC CURRENT TRANSVERSELY THROUGH THE MEMBRANES AND CHAMBERS; THE IMPROVEMENT COMPRISING A PLURALITY OF SPACED CENTERED STRAPS IN SAID SPACER MEMBERS RUNNING ACROSS SAID CHAMBERS IN PARALLED TO THE 